1. Field of the Invention
The present invention relates to a Raman amplifier capable of being used for amplification of optical signal in various optical communication systems, and optical repeater and Raman amplification method using such a Raman amplifier, and more particularly, it relates to Raman amplifier, optical repeater and Raman amplification method suitable for amplification of wavelength division multiplexing signals.
2. Related Background Art
Almost all of optical amplifiers used with present optical fiber communication systems are rare earth doped fiber amplifiers. Particularly, erbium doped optical fiber amplifier (referred to as “EDFA” hereinafter) using Er (erbium) doped fibers have been used frequently. However, a practical gain wavelength band of the EDFA has a range from about 1530 nm to 1610 nm (refer to “Electron. Lett,” vol. 33, no. 23, pp. 1967-1968). Further, the EDFA includes gain having wavelength dependency, and, thus, when it is used with wavelength division multiplexing signals, difference in gain is generated in dependence upon a wavelength of optical signal. FIG. 23 shows an example of gain wavelength dependency of the EDFA. Particularly, in wavelength bands smaller than 1540 nm and greater than 1560 nm, change in gain regarding the wavelength is great. Accordingly, in order to obtain given gain (in almost cases, gain deviation is within 1 dB) in the entire band including such wavelength, a gain flattening filter is used.
The gain flattening filter is a filter designed so that loss is increased in a wavelength having great gain, and the loss profile has a shape substantially the same as that of the gain profile. However, As shown in FIG. 24, in the EDFA, when magnitude of average gain is changed, since the gain profile is also changed as shown by curves a, b and c, in this case, the optimum loss profile of the gain flattening filter is also changed. Accordingly, when the flattening is realized by a gain flattening filter having fixed loss profile, if the gain of the EDFA is changed, the flatness will be worsened.
On the other hand, among optical amplifiers, there is an amplifier referred to as a Raman amplifier utilizing Raman scattering of an optical fiber (refer to “Nonlinear Fiber Optics, Academic Press). The Raman amplifier has peak gain in frequency smaller than frequency of pumping light by about 13 THz. In the following description, it is assumed that pumping light having 1400 nm band is used, and the frequency smaller by about 13 THz will be expressed as wavelength longer by about 100 nm. FIG. 25 shows wavelength dependency of gain when pumping light having central wavelength of 1450 nm is used. In this case, peak of gain is 1550 nm and a band width within gain deviation of 1 dB is about 20 nm. Since the Raman amplifier can amplify any wavelength so long as an pumping light source can be prepared, application of the Raman amplifier to a wavelength band which could not be amplified by the EDFA has mainly be investigated. On the other hand, the Raman amplifier has not been used in the gain band of the EDFA, since the Raman amplifier requires greater pump power in order to obtain the same gain as that of the EDFA. When the pumping light having great power is input to a fiber to increase the gain, stimulated Brillouin scattering may be generated. Increase amplification noise caused by the stimulated Brillouin scattering is one of the problems which makes it difficult to use the Raman amplifier. the Japanese Patent Laid-open No. 2-12986 (1990) discloses an example of a technique for suppressing the stimulated Brillouin scattering in the Raman amplifier.
Further, the Raman amplifier has polarization dependency of gain and amplifies only a component (among polarized wave components) coincided with the polarized wave of the pumping light. Accordingly, it is required for reducing unstability of gain due to polarization dependency, and, to this end, it is considered that a polarization maintaining fiber is used as a fiber for amplifying or an pumping light source having random polarization condition.
Furthermore, enlargement of the gain band is required in the Raman amplifier. To this end, Japanese Patent Publication No. 7-99787 (1995) teaches in FIG. 4 that the pumping light is multiplexed with appropriate wavelength interval. However, this patent does not disclose concrete values of the wavelength interval. According to a document (K. Rottwitt, OFC98, PD-6), a Raman amplifier using a plurality of pumping lights having different wavelengths was reported; however, attempt in the viewpoint of the fact that the gain deviation is reduced below 1 dB was not considered.
On the other hand, there is an optical repeater for simultaneously compensating for transmission loss and chromatic dispersion in an optical fiber transmission line, which optical repeater is constituted by combination of an Er doped fiber amplifier (EDFA) and a dispersion compensating fiber (DCF). FIG. 46 shows a conventional example in which a dispersion compensating fiber A is located between two Er doped fiber amplifiers B and C. The first Er doped fiber amplifier B serves to amplify optical signal having low level to a relatively high level and has excellent noise property. The second Er doped fiber amplifier C serves to amplify the optical signal attenuated in the dispersion compensating fiber A to the high level again and has a high output level.
By the way, on designing the optical repeater, it is required that a repeater input level, a repeater output level and a dispersion compensating amount (loss in the dispersion compensating fiber A) be set properly, and, there is limitation that the input level of the dispersion compensating fiber A has an upper limit, because, when the input power to the dispersion compensating fiber A is increased, influence of non-linear effect in the dispersion compensating fiber A is also increased, thereby deteriorating the transmission wave form considerably. The upper limit value of the input power to the dispersion compensating fiber A is determined by self phase modulation (SPM) in one wave transmission and by cross phase modulation (XPM) in WDM transmission. Thus, regarding the optical repeater, an optical repeater having excellent gain flatness and noise property must be designed in consideration of the several variable factors.
FIG. 47 shows a signal level diagram in the repeater. Gain G1 [dB] of the first Er doped fiber amplifier B is set to a difference between an input level Pin [dB] of the repeater and an input upper limit value Pd [dB] to the dispersion compensating fiber A. Gain G2 [dB] of the second Er doped fiber amplifier C is set to (Gr+Ld−G1) [dB] from loss Ld [dB] in the dispersion compensating fiber A, gain Gr [dB] of the repeater and the gain G1 [dB] of the first Er doped fiber amplifier B. Since these design parameters are varied for each system, the values G1 [dB] and G2 [dB] are varied for each system, and, accordingly, the Er doped fiber amplifiers B, C must be re-designed for each system. The noise property in such a system is deeply associated with the loss Ld [dB] in the dispersion compensating fiber A, and it is known that the greater the loss the more the noise property is worsened. Further, at present, a gap from the designed value in loss in the transmission line and the loss in the dispersion compensating fiber A are offset by changing the gains of Er doped fiber amplifiers B, C. In this method, the gain of Er doped fiber amplifier B and C are off the designed value, thus the gain flatness is worsened. A variable attenuator may be used to offset the gap from the designed value of loss. In this method, although the gain flatness is not changed, an additional insertion loss worsens the noise property.
In the optical fiber communication system, although the Er doped optical fiber amplifiers have widely been used, the Er doped optical fiber amplifier also arises several problems. Further, the Raman amplifier also has problems that, since output of ordinary semiconductor laser is about 100 to 200 mW, gain obtained is relatively small, and that the gain is sensitive to change in power or wavelength of the pumping light. So that, when a semiconductor laser of Fabry-Perot type having relatively high output is used, noise due to gain fluctuation caused by its mode hopping becomes noticeable, and that, when the magnitude of the gain is adjusted, although drive current of the pumping laser must be changed, if the drive current is changed, since the fluctuation in the central wavelength is about 15 nm at the maximum, the wavelength dependency of gain will be greatly changed. Further, such shifting of the central wavelength is not preferable because such shifting causes change in joining loss of a WDM coupler for multiplexing the pumping light. In addition, the optical repeater also has a problem that the Er doped optical fiber amplifiers B, C must be re-designed for each system. Further, the deterioration of the noise property due to insertion of the dispersion compensating fiber is hard to be eliminated in the present systems.
In a Raman amplification method for amplifying optical signal by using a stimulated Raman scattering phenomenon, a communication optical fiber is used as an optical fiber acting as an amplifying medium, and, in a distributed amplifying system, a wavelength of pumping light and a wavelength of the optical signal are arranged in 1400 nm-1600 nm band having low loss and low wavelength dependency within a wide band of the communication optical fiber. In this case, regarding the loss of wavelength dependency of the optical fiber as the amplifying medium, a difference between maximum and minimum values is below about 0.2 dB/km in the above band, even in consideration of loss caused by hydroxyl ion (OH) having peak at 1380 nm. Further, even if each pumping powers in a multi-wavelength pumping system are not differentiated according to the wavelength dependence of the loss, the gain of the signals amplified by the pumping lights are substantially the same, there is no problem in practical use.
On the other hand, in a Raman amplifier operating as a discrete amplifier such as EDFA (rare earth doped fiber amplifier) it is necessary to pay attention to the package of the amplifier fiber, for a length of the fiber is about 10 km to about several tens of kilometers in order to obtain the required gain. Thus, it is convenient that the length of the fiber is minimized as less as possible. Although the length of the fiber can be shortened by using an optical fiber having great non-linearity, in the optical fiber having great non-linearity, it is difficult to reduce the transmission loss caused by (OH) generally having a band of 1380 nm, and Rayleigh scattering coefficient becomes great considerably in comparison with the communication fiber, with the result that the difference between the maximum and minimum values of fiber loss within the above-mentioned wavelength range becomes very great such as 1.5 to 10 dB/km. This means that, when the optical fiber having length of 3 km is used, the loss difference due to the wavelength of the pumping light becomes 4.5 dB to 30 dB. Thus, the wavelength division multiplexing signals cannot be uniformly amplified by using the pumping lights having the same intensities.
As one of means for multiplexing the number of pumping lights, there is a wavelength combiner of Mach-Zehnder interferometer type. Since the Mach-Zehnder interferometer has periodical response property regarding frequency, the wavelength of the pumping light must be selected among wavelengths having equal intervals in frequency. Accordingly, the wavelength combiner of Mach-Zehnder interferometer type has limitation in degrees of freedom of wavelength setting, but has an advantage that, when a device of waveguide type or fiber fusion type, if the number of wavelength division multiplexing is increased, insertion loss is not changed substantially.